Hello! Are OCs originally made for DnD or another TTRPG system considered "OC" (appable) or "Fandom OC" (unappable)?

Does it make a difference if the OC's campaign world is completely original, versus their campaign having been in a "canon" setting such as DnD's Forgotten Realms (assuming in good faith that the character has no ties to major canon characters or events)?

If such an OC is unappable, would it still be okay if they were reskinned as an AU character?

Thanks in advance!